1. Field of the Invention
The present invention relates generally to a ball bonding method on a chip and more particularly to the configuration of the ball or bump on the second bonding pad such that all of the balls or bumps have a uniform shape and size of vertical tips.
2. Description of the Related Art
For many years wire bonding prevented damage to the bonding pad from the capillary hitting the bonding pad directly by forming a ball on the bonding pad prior to wire bonding so that the capillary would hit the ball during wire bonding. The ball-forming process comprises the following steps: ball formation on a capillary; thermocompression bonding on a bonding pad; capillary horizontal movement for stitch bonding; and capillary vertical movement for wire cutting.
U.S. Pat. No. 5,641,079, issued on Jul. 12, 1994 to Mathew et al., discloses method for bond wire connection and a conductive bump thereof prior to stitch bonding. As shown in FIG. 1, U.S. Pat. No. 5,641,079, a substrate 100 is attached to a chip 110 which has a second bond point as well as a bonding pad 111. A capillary 120 of the thermosonic or thermocompression ball bonding tool forms a bump on each bonding pad 111 and connections are made to the bonding pads 111 of the chip 110 by means of stitching bonding in a way which does not damage the chip 110. A bump 121 is formed at the free end of the wire in the capillary 120. Then, the clamp 130 is closed to hold the wire.
As shown in FIG. 2, the capillary 120 is moved down in a vertical direction to the second bonding point so as to let the bottom of the bump 121 weld to the bonding pad 111. Then, the capillary 120 does not hit the bonding pad 111 and the clamp 130 of the capillary 120 is opened to release the wire.
As shown in FIG. 3, the capillary 120 is moved in a horizontal direction thereby moving the tip as well as connection part 122 to the side. Then the capillary 120 is moved down again in a vertical direction so as to press the tip 122 of the bump 121 to form a thin neck. Then, the capillary 120 is opened to hold the wire.
As shown in FIGS. 4 and 5, the capillary 120 is moved up in a vertical direction and then the clamp 130 is closed thereby cutting the thin neck portion which connects to the tip 122 of the bump 121 such that the bump 121 is left on the bonding pad 111. Because the bump 121 on the bonding pad 111 is pressed by the capillary 120, the tip 122 of the bump 121 is formed in a particular direction and collapse shape. The direction and collapse shape of the tip 122 determines the parameter (e.g., the movement) of the wire bonding tool for the second bonding so as to increase quality and reliability of products. However, the direction and collapse shape of the tip 122 of each bump 121 are various and parameter adjustment of each wire bonding is impossible in this circumstance. Thus only one set of parameters can be used in each wire bond, and we are unable to control the quality and reliability of wire bonding of products under these parameter controls.
The present invention intends to provide a ball with a uniform shape and height for wire bonding at a second bonding point to decrease the variability of the ball and increase work window range in such a way as to mitigate and overcome the above problem.